1. Field of the Invention
The present invention relates to a feeding device for delivering low viscosity paste material under pressure, and more particularly, to an improved manual dispensing mechanism therefor.
2. Description of the Prior Art
There exist various types of hand-operated paste material dispensing devices of the caulking gun variety which include a plunger adapted to advance axially through a cylinder to which the end of a paste material cartridge is attached. The plunger is advanced by a trigger-type lever pivoted to a pistol grip, and a ratchet device is mounted to the end of the lever for engaging a rod forming part of the plunger.
Such a dispensing device is advantageously used in the delivery of bone cement into cavities formed in a bone to which a prosthesis is to be mounted. An example of such an apparatus and method is described in U.S. Pat. No. 4,338,925, issued July 13, 1982 to Jo Miller, one of the co-inventors of the present invention. This patent is incorporated herein by reference. The Miller patent describes a high pressure injection gun including a housing axially supporting a plunger and rod. A hardened steel disc with a central opening rests on the rod. Contraction of the lever against the pistol grip causes the disc to tilt, lock onto the rod, and move the rod forward while compressing an axially aligned return spring. When the trigger is released, the return spring presses against the disc, releasing it from engagement on the rod and forcing it to slide on the rod to its initial position while returning the lever.
It has been found that there is a considerable amount of lost-motion in tilting the disc. For instance, one third of the travel of the lever can be lost-motion. Furthermore, when the edges of the disc defining the central opening have been worn, the disc can slip on the rod, which can be a serious situation in the middle of a surgical operation.
Other injection guns exist, such as the type described in U.S. Pat. No. 4,406,654, issued Sept. 27, 1983 to Robert L. Bristow. In this patent, spring mounted pawls are used which engage notches in the rod. One of the pawls is pivotally mounted to the lever. Since the rod is of relatively small diameter, the lever beyond the pivot point must be longer for the same amount of travel, and thus the pressure applied tends to be less. Although the ratchet and notch principle used in Bristow and in a caulking gun is an improvement, the mechanical advantage provided by the disc in the Miller U.S. Pat. No. 4,338,925 is lost. In the Miller patent, the disc, when engaged on the rod, effectively increases the diameter of the rod. Only a short extent of the lever is necessary above the pivot point to provide a low volume, high pressure shot of cement.